Method for annealing or hardening of metals

ABSTRACT

The invention relates to a method for annealing or hardening of a metal or an alloy in a controlled furnace atmosphere, wherein said furnace atmosphere is a hydrogen-free atmosphere comprising nitrogen and carbon monoxide.

The invention relates to a method for annealing or hardening of a metal or an alloy in a controlled furnace atmosphere.

The composition, function and control of the furnace atmosphere are of crucial importance for the result of all heat treatments.

In the following, the term annealing is used for heat treatment of metals or alloys which requires a controlled atmosphere, and where the aim is to produce certain microstructures and properties. Annealing is done both in steel and nonferrous milling plants.

In the manufacturing industries annealing furnaces have a wide range of sizes and designs: from a small box furnace in a tool room, to a big continuous isothermal annealing furnace in the automotive transmission workshop, from the advanced vacuum furnace in the aerospace industry, to air filled furnaces for lower demand industries.

Hardening is a heat treatment procedure for steels with the aim to produce a martensitic or bainitic microstructure. Hardening is normally described separately from annealing but the furnace atmosphere requirements are similar for both processes. Hardening involves the steps of heating up to a temperature above the temperature where the steel is austenitic, followed by a fast quench. The austenite will transform upon quenching to the hard microstructure martensite or bainite if the quenching rate is fast enough.

To avoid the negative effects of oxidation, carburizing, decarburization or nitriding during annealing or hardening, the atmosphere must be controlled with respect to the annealed or hardened alloy. The carbon potential of a furnace atmosphere is equal to the carbon content that pure iron would have in equilibrium with the atmosphere. The carbon activity (aC) of a furnace atmosphere is the carbon content a metal or alloy would have compared to the reference, graphite, defined as being equal to aC=1. Both the carbon activity and the carbon potential in heat treatment processes affect the final properties of the metal/alloy in many ways.

in order to avoid the creation of metal oxides on the surface of the heat treated metal or alloy the heat treatment process is often carried out in a reducing atmosphere. For example, a heat treatment atmosphere for annealing of copper wire coils might contain nitrogen with 3% hydrogen.

Besides the described synthetic nitrogen-hydrogen atmosphere, today the atmosphere is often based on a combination of nitrogen and endogas. If the endogas is made from natural gas it may contain up to 40 vol % of hydrogen, some carbon monoxide (ca 20 vol %), carbon dioxide and water (ca 0.3-1 vol %) with the remainder being nitrogen.

In neutral annealing and neutral hardening, e.g. hot forming of steel parts, a reducing atmosphere has to be used in order to avoid oxidation of the steel surface. A typical reducing agent is hydrogen. However, especially when a high-strength steel is subjected to a hydrogen containing atmosphere hydrogen embrittlement may occur, that is the steel becomes brittle and might crack.

Thus it is an object of the invention to develop a controlled furnace atmosphere for annealing or hardening of metals or alloys.

This object is achieved by a method for annealing or hardening of a metal or an alloy in a controlled furnace atmosphere, wherein said furnace atmosphere is a hydrogen-free atmosphere comprising nitrogen and carbon monoxide.

According to the invention a furnace atmosphere is used which is essentially free of hydrogen and which comprises nitrogen and carbon monoxide. The concentration of carbon monoxide in nitrogen could be between 0.1 and 99 vol %. The proposed atmosphere has no or only low driving force for de-carburization.

The invention is used in annealing and hardening of metals and alloys, such as steel, aluminium, copper or brass. The term annealing shall in particular cover the methods of recrystallization, bright annealing, stress relieving, solution annealing and precipitation hardening, isothermal annealing, soft annealing and normalizing.

Especially in hardening processes the metal or alloy is rapidly cooled, especially by gas cooling, after the heat treatment process. The cooling is preferably achieved by quenching the metal parts by means of a cold protective gas. Thereby cooling rates of up to 50° C./sec are achievable. It has been found that in this way a hard martensitic microstructure is achieved.

The inventive furnace atmosphere is in thermodynamic equilibrium. Thus, it is possible to implement a process control using an external heated oxygen probe or a gas analyser measuring carbon dioxide in combination with measurements of the carbon monoxide level and the process temperature.

The invention is preferably used for annealing or hardening of metals and alloys of any kind, in particular metallic material comprising one or more of iron, steel, aluminium, copper, brass, bronze or hard metals. Further alloying elements such as chromium, manganese, silicon, nickel, molybdenum, cobalt or tungsten may be added.

The invention provides an atmosphere for neutral annealing and neutral hardening. That means, there are essentially no net reactions between the atmosphere and the metal. The inventive atmosphere is neutral with respect to carburization, that is undesired de-carburization as well as carburization are avoided. Metal oxides, in particular surface metal oxides, are reduced and oxidation is prevented.

The inventive atmosphere may be advantageously produced by one of the following methods:

-   -   Removal of hydrogen from endogas:     -   In order to create the inventive atmosphere hydrogen is removed         from the endogas. This is preferably achieved by using         adsorption techniques, in particular a PSA (pressure swing         adsorption) process.     -   Removal of hydrogen from syngas:     -   Syngas or synthesis gas, is the name given to gases of varying         composition that are generated in coal gasification and some         types of waste-to-energy gasification facilities Syngas consists         primarily of carbon monoxide and hydrogen. By removing the         hydrogen from the syngas an inventive atmosphere is created         which has a high carbon monoxide concentration.     -   Removal of hydrogen from cracked methanol since cracked methanol         could be regarded as a syngas with the composition of 33% CO and         67% H₂.     -   Production of carbon monoxide with added air over a heated bed         of doped graphite:     -   Air or nitrogen with a quality containing residual oxygen levels         up to 3% is used and the contained oxygen is caused to react to         carbon monoxide inside the furnace over a graphite or coal bed         or in an external coal filled reactor.     -   Production of carbon monoxide by dissociating formic acid         injected into a heated reactor filled with sulphuric acid or         phosphoric acid. The formed carbon monoxide is then dried from         water and scrubbed to reach neutral pH-value.

Since the inventive atmosphere is hydrogen free the above mentioned problems with hydrogen embrittlement are avoided. Thus the invention is particularly useful in hardening processes where the parts are not tempered after quenching which could take place in cooled tools, submersion in water, oil polymer, molten salt or in gas. Examples of such processes are hot forming of steel or press hardening of steel. Oxidation of the steel surface is prevented by the inventive addition of carbon monoxide to the atmosphere which has a reducing effect.

EXAMPLES

The invention provides a solution for a reducing controlled atmosphere for annealing or hardening coated steel, which in hydrogen containing atmosphere could dissolve hydrogen into the steel.

Another example of processes where the invention is used with advantage is in manufacturing of fasteners, e.g. screws, bolts, nails, nuts. These products are subjected to a high static load during the lifetime, a condition where atomic dissolved hydrogen could be a potential risk especially when tempering of the martensite is done at low temperature.

Another example relates to bainite hardened steels which are quenched in molten salt or hot gas. The inventive hydrogen free atmosphere could positively affect the austenitising process, e.g. in the production of bearing steels. 

1. Method for annealing or hardening of a metal or an alloy in a furnace atmosphere, comprising providing a hydrogen-free atmosphere comprising nitrogen and carbon monoxide.
 2. Method according to claim 1, wherein said furnace atmosphere comprises a carbon containing enrichment gas.
 3. Method according to claim 2, wherein said enrichment gas is selected from acetylene, propane and methane.
 4. Method according to claim 1, comprising removing hydrogen from syngas, from endogas or from cracked methanol for producing said furnace atmosphere.
 5. Method according to claim 1, comprising reacting nitrogen containing less than 10% oxygen over graphite for producing said furnace atmosphere.
 6. Method according to claim 1, comprising injecting formic acid into a heated reactor filled with sulphuric acid or phosphoric acid and dissociating said formic acid for forming the carbon monoxide.
 7. Method according to claim 1, comprising measuring carbon monoxide and temperature of said furnace atmosphere by a heated external oxygen probe or a carbon dioxide gas analyser for controlling the annealing or hardening.
 8. Method according to claim 1, wherein said furnace atmosphere comprises between 1% and 5% CO, between 90% and 99% N₂ and between 0.05% and 1% hydrocarbon gas.
 9. Method according to claim 1 comprising subjecting said metal or alloy to a hardening process comprising heating said metal or alloy and quenching said metal or alloy.
 10. Method according to claim 1, further comprising manufacturing fasteners made of metal or alloy in said furnace atmosphere. 